1. Field of the Invention
The present invention relates to a lock-up control apparatus for an automatic transmission of a vehicle, for controlling a lock-up state of a torque converter wherein its input and output elements are directly connected to each other during a coasting drive including deceleration of the vehicle.
2. Description of the Related Art
It is a recent trend for automatic transmissions to adopt a lock-up system which can turn a torque converter into a lock-up state in which its input and output elements are directly connected to each other during the vehicle drive in a lock-up region in which a torque increasing function or a torque fluctuation absorbing function by the torque converter is not required, in order to improve the transmission efficiency and thereby enhance the fuel economy.
This type of automatic transmission is fully described, for example, in "SERVICE MANUAL FOR NISSAN RE4R01A-TYPE FULL-RANGE ELECTRONICALLY-CONTROLLED AUTOMATIC TRANSMISSION" issued by Nissan Motor Co., Ltd., the assignee of the entire right and interest relative to the present application. To carry out a lock-up control of such automatic transmission, as exemplarily shown in FIG. 21 in which a lock-up ON line is denoted by a double-dotted line and a lock-up OFF line is denoted by a single-dotted line, it has been a conventional practice to judge the vehicle driving state in either of lock-up region (L/U) or converter region (C/V) determined by a throttle opening TH (engine operation load) and a vehicle speed V, and to apply a lock-up clutch in the lock-up region to turn the torque converter into the lock-up state in which the input and output elements are directly connected or to release the lock-up clutch in the converter region to turn the torque converter into the converter state in which the direct connection is released, in accordance with the result of judgment.
In order to enhance the fuel economy by locking up the torque converter, it is required to enlarge the lock-up region so that the torque converter can be locked up in as low load driving condition and as low vehicle speed as possible. Thus, the lock-up region is determined as shown in FIG. 21, for example.
Since the power from an engine is unnecessary during the coasting drive including the deceleration operation with the accelerator pedal released, there is known a fuel cut device which stops the fuel supply to the engine during the coasting drive thereby to improve the fuel economy of the vehicle. The fuel cut device stops the fuel cut to restart the fuel supply (fuel recovery) when the engine speed is lowered to a predetermined speed (fuel recovery engine speed) in order to prevent the engine from stalling. With such a fuel cut device, the fuel economy can be effectively improved particularly when the reduction of the engine speed during the coasting drive is delayed to prolong the fuel cut time. It is therefore a general practice, in a vehicle having an engine with a fuel cut device, to turn the torque converter into the lock-up state during the coasting drive in which the throttle opening TH is 0/8, as shown in FIG. 21.
In automatic transmissions wherein the torque converter during the coasting drive of the vehicle is turned into the lock-up state in which the input and output elements are directly connected with each other, when a braking of the vehicle is performed in the lock-up state of the torque converter, by depressing a brake pedal during the drive in the lock-up state, the rotation of wheels tends to be suddenly stopped. Such a tendency is significant particularly in the case of braking on a road with a low friction. However, the lock-up state of the torque converter cannot be swiftly released in response to the sudden stop of the wheels, due to a relatively large response delay of the torque converter. Thus, there may be instances wherein the engine in a drive-connection with the wheels undergoes stalling.
JP-A-4-370465 discloses a proposal wherein a relative rotation between the input and output elements of the torque converter is allowed by switching the torque converter from the lock-up state to a slip control state in which a slip occurs when the brake pedal is depressed during the coasting drive, and the lock-up of the torque converter is released upon sudden deceleration of the vehicle when the brake pedal is further depressed.
While such a proposal serves to solve the problem of the engine stalling due to the response delay of the lock-up release, a further problem may arise from the arrangement wherein the torque converter is turned into the slip state immediately after the coasting drive starts even when the brake is not applied suddenly. That is to say, the slip of the torque converter provokes a reduction in the engine speed corresponding to the slip amount and the fuel cut must be stopped to initiate fuel recovery at an earlier instant, thereby making it difficult to achieve a satisfactory fuel economy. In other words, the above-described proposal may not be a satisfactory solution to the extent that the engine stalling can be prevented only at a sacrifice of an improved fuel economy to be achieved by the fuel cut.